In cellular radio systems, background noise, which is modelled as AWGN (Additive White Gaussian Noise) and various interferences, such as interference signals or fading multipath-propagated signals may be summed to a signal on a radio channel. One way to improve the reliability of data transmission is to use spread spectrum technique. The spread spectrum methods are data transmission methods, in which the signal is transmitted on a much broader bandwidth than the minimum bandwidth required for data transmission. FIG. 1 shows the schematic structure of a spread spectrum system. The signal spectrum is spread in a transmitter on the frequency level after information modulation 100 by using a pseudo-random spreading code 102, which is independent of the original, generally digital, information signal. In a receiver, the signal is despread by multiplying the signal again by the spreading code 104, whereafter the signal is demodulated 106. The performance of the spread spectrum system against interference signals is often detected by detection amplification, which represents improved signal-to-noise ratio in the receiver. Because the interference does not generally correlate with the spreading code, the despreading of the spectrum in the receiver is spreading modulation in view of interference, i.e. the interference signal resembles common noise on the channel, and as the information signal is a narrowband signal after despreading, most of the interference can be filtered off, which provides detection amplification.
A method, in which all users transmit simultaneously using the same frequency band and in which the signals of different users can be distinguished in receivers on the basis of the spreading code by using a specific spreading code on each communication connection between a base station and a mobile station, is called the code division multiple access method CDMA. Cross correlation properties of the spreading codes will be minimized by selecting the codes to be used such that they are mutually orthogonal. In WCDMA (Wideband Code Division Multiple Access) receivers the correlators are in synchronization with the signal to be identified on the basis of the spreading code. The signals that have been multiplied by some other code in the transmission stage, do not correlate, in an ideal case, with the code used for multiplication and thus they will remain in broadband format. Hence, they appear as noise in view of the desired signal. The object is thus to detect the desired user signal among a plurality of interfering signals. In practice, the spreading codes are not uncorrelated, because the number of mutually orthogonal codes is insufficient, and therefore the signals of other users produce error in the detection of the desired signal by distorting the received signal non-linearly.
In the future, the same or an adjacent frequency band can be simultaneously used by a plurality of cellular radio systems, typically a narrowband system, such as the GSM (Global system for Mobile Communications) or the PDC (Pacific Digital Cellular System), and a broadband system, such as the WCDMA system). In simultaneous use of the same frequency or simultaneous use of an adjacent frequency, the broadband system is typically used for high-rate data transmission and the narrowband system for speech transmission. FIG. 2 shows a typical overlay use situation. A base station 204 of the narrowband NB system forms there a narrowband cell 200 and a base station 206 of the WCDMA system forms a broadband cell 202. Narrowband transmission 210 interferes with receivers of both the broadband base station and subscriber terminal 208. In this kind of simultaneous use, improved performance provided by detection amplification of the broadband system is not always sufficient, but the broadband signal having considerably higher power can significantly deteriorate the receiver performance of the broadband system, such as the WCDMA system, or even block it. The WCDMA system is interference limited, so that interference reduces either capacity or coverage area. It appears from FIG. 3 how the narrowband signal 302 power is considerably higher than the broadband signal 300 power, which is close to the power level of background noise. Since the broadband signal resembles AWGN noise on the channel, it does not cause extra interference in the receiver of the narrowband system. In addition to the power level of the interfering signal, many other factors, such as the duration of interference, the modulation method and encoding used, affect the performance, i.e. tolerance to various interference, of the cellular radio systems.
Methods have been presented attempting to reduce degradation in performance caused to the receiver of the broadband system by the narrowband system in the simultaneous use of the same frequency band. In known methods, interference cancellation generally requires that various mathematical transformation methods of signals be used for regenerating the signal and the interference, which takes time and computing capacity, or, the interference cancellation is implemented in baseband frequency parts, even though it should be implemented at the earliest possible stage. The known methods, in which interference cancellation is implemented in radio frequency parts and in which the signal and the interference need not be regenerated, are based on available advance information on the interfering system. One method of this kind is disclosed in “Experimental WCDMA data overlay of GSM network” by Noël and Widdowson, British Telecom Res. Labs., Electronic Letters, Apr. 15, 1999, Vol. 35, Issue 8, which is incorporated herein by reference. Said publication describes a method which attempts to cancel interference caused by a narrowband signal as close to the radio frequency parts as possible to minimize the effects of interference by using a band-stop filter tuned to a predetermined frequency. The described method has a drawback that the frequency used by the interfering system must be known in order to be able to tune the band-stop filter. Often, this is only possible if the broadband system and the interfering narrowband system are used by the same operator. In addition, the performance of narrowband systems is often improved, particularly in multipath propagation environments, by using frequency hopping which allows the interfering narrowband signal to change the frequency it uses in the frequency band of the broadband system. In this case, tuning the band-stop filter in advance onto one frequency does not improve the performance sufficiently. A narrow band-stop filter is not sufficiently efficient either for cancelling interference caused by background noise peaks, such as microwave ovens or car starting systems, generated by a plurality of other man-made interference signals. Typically, also in these situations, signals, which cause interference in the receiver band of the broadband system, vary as a function of time, and therefore it is not possible to know their frequencies in advance in order for the filter to be tuned.